If you want to design an aircraft for use in FS2004/ CFS2 then you need to use a design program like Gmax (freeware)or Abacus FSDS2.24(Payware). These are 3D "CAD" packages which allow you to prepare a visual 3Dmodel.  When the model is ready, or during construction, you'll need a couple of programs like Paint Shop Pro and Martin Wright's DXTBmp to be able to paint them.

You'll also need an .AIR file, this can be one taken from a similar aircraft and modified to suit. The .AIR file is the file which controls the way an aircraft actually flies in the simulator.

Check out the threads here and the tut's already mentioned in the FAQ's.

Looking at the components of the aircraft folders in flight sim will give you a good idea of the end products.  There you will find folders for the model, the panel, the sounds, and the textures.  Other items found in each folder will be the aircraft.cfg and nnn.air files, and the reference and checklist.

Let's look briefly at each folder and what is contained.

1.  Model contains the xxx.mdl file which is the compiled output of the 3D design program for the visual model exterior and virtual cockpit and interior model.  Gmax is a freeware 3D design program that comes with FS9 that can be used to design the visual model.

2.  Panel contains the 2D and 3D virtual cockpit panel and gauge information described by the panel.cfg file.  The gauges and bitmaps here make of the panel(s) but gauges may also reside in the FS Gauge folder.  The assembly of gauges and panel design can be done with several programs like cfgedit but new gauges must be written in C++ or xml.  These contain bmps and program code for the gauge logic.


3.  Sounds contain the sound.cfg which defines how the sound .wav files are to be mixed and heard.  Though the sound.cfg can be edited with notepad, a more visually comprehensive mixing program is needed to make sense of it all.  ACM may help here.  http://www.aircraftmanager.com/


4.  Textures contain the graphic bitmaps for the exterior and interior model paint schemes, light maps, and are usually stored in  various formats so that extra information can be stored that control reflectivity, transparency, night lighting, or backlighting.  Programs like Photoshop or Paint Shop Pro are typically used for complex layered schemes.

There may be one or more of each of these folders in an aircraft folder.

Other required files are the aircraft.cfg which is used in conjunction with the xxx.air file to provide the flight model for the aircraft.  The .air file can be edited/modified with programs like AirEd found here: http://perso.wanadoo.fr/hsors/FS_Soft/index.html

You will also find lots of technical reference information there.

The aircraft.cfg may be edited with Notepad or Wordpad.  

The reference and checklist files while optional provide information to help new virtual pilots understand the requirements of flying the aircraft correctly.  These are also edited with Notepad or an XML editor.

Effects for the aircraft defined in the aircraft.cfg are stored in the FS/Effects folder.  These consist of logic and bmps and can be edited in Notepad or Wordpad.

MS has provided basic SDK's that give you a sense of what these things are about and generally how to use, modify, or create new ones.

http://www.microsoft.com/games/flightsimulator/fs2004_downloads_sdk.asp